1. Field of the Invention
The present invention relates to a surface acoustic wave device used as a filter or a resonator.
2. Description of the Related Art
Advancements in mobile communication technologies in recent years have been causing communication equipment to become much more compact and operate at higher frequencies. Such equipment requires oscillators and high-frequency filters as indispensable components, and often includes surface acoustic wave devices.
Conventional surface acoustic wave devices, such as surface acoustic wave filters and surface acoustic wave resonators, are made by forming interdigital transducers on a piezoelectric substrate such as lithium niobate or lithium tantalate, whereon the surface acoustic wave is generated by applying an alternate electric field to the interdigital transducers. A surface acoustic wave device used in mobile communication equipment must have good operational characteristics in the high frequency range. High-frequency characteristics of a surface acoustic wave is evaluated as an insertion loss and the temperature dependence thereof in the case of a filter. In the case of a resonator, high-frequency characteristics are evaluated as a resonation Q value which corresponds to the inverse of loss, a ratio of resonance to anti-resonance (capacity ratio), and the temperature dependances thereof. The capacity ratio has a direct effect on frequency pass-band. The insertion loss, the resonation Q, and the capacity ratio depend on the electromechanical coupling factor of the piezoelectric material. The temperature dependence of these parameters have a relation to the temperature dependence of the acoustic velocity of the piezoelectric material to be used.
With respect to the production process of surface acoustic wave devices, the line width of an interdigital transducer depends on the sound velocity of a piezoelectric substrate, therefore the sound velocity of the piezoelectric substrate is also important in order to facilitate a fine patterning process such as photolithography.
The electromechanical coupling factor, the temperature dependence, and the sound velocity are largely dependent on the type and crystal orientation of the material used. In the case of lithium niobate, under the conditions of 64-degree Y-cut and X-propagation, the electromechanical coupling factor is 11.3%, the temperature dependence is 70 ppm/.degree. C., and the sound velocity is 4,742 m/sec. Under the conditions of 128-degree Y-cut and X-propagation, the electromechanical coupling factor is 5.5%, the temperature dependence is 75 ppm/.degree. C., and the sound velocity is 3,980 m/sec. In the case of lithium tantalate, under the conditions of 36-degree Y-cut and X-propagation, the electromechanical coupling factor is 5.0%, the temperature dependence is 30 ppm/.degree. C., and the sound velocity is 4,160 m/sec. With quartz, under the conditions of 42.5-degree Y-cut and X-propagation, the electromechanical coupling factor is 0.15%, the temperature dependence is 0 ppm/.degree. C., and the sound velocity is 3,158 m/sec. With lithium borate, under the conditions of 45-degree X-cut, the electromechanical coupling factor is 1.0%, and the sound velocity is about 3,401 m/sec.
From the view point of the electromechanical coupling factor, generally, it is preferable to use lithium niobate. However, lithium niobate is inferior in temperature dependence to quartz, etc. Quartz has a very small temperature dependence, but has a small electromechanical coupling factor. As the sound velocity is higher, interdigital transducers in a resonator or filter for a high frequency can have a wider line width. From the view point of the sound velocity, therefore, it is preferable to use lithium niobate of 64-degree Y-cut and X-propagation.
From the view point of the flexibility of design, it is preferable to use a material which has a large electromechanical coupling factor, a small temperature dependence, and a high sound velocity. However, the above-mentioned materials are insufficient for satisfying these requirements.
In a prior art piezoelectric substrate composed of a single material, the combination of the electromechanical coupling factor and the temperature dependence is limited in number, and hence the flexibility of design is low. Furthermore, there are problems in that a material which has a large electromechanical coupling factor and a small temperature dependence has not been found, and that a piezoelectric substrate of a high sound velocity has not been developed.
In order to solve these problems, surface acoustic wave devices of a lamination structure have been developed. For example, a configuration has been reported in which piezoelectric films are laminated on a non-piezoelectric made of a material of a high sound velocity such as sapphire or diamond, thereby obtaining a surface acoustic wave substrate of a high sound velocity (for example, Japanese Laid-Open Patent publication No. 64-62,911). In such a device, as a piezoelectric film, a thin film made of ZnO or AlN is formed of by a thin film forming technique such as a sputtering method or a chemical vapor deposition (CVD) method.
A lamination structure of ZnO and lithium niobate which constitute a lamination of piezoelectric materials is reported by A. Armstrong et al. (Proc. 1972 IEEE Ultrasonics Symp. (IEEE, New York, 1972) p. 370). According to this configuration, a surface acoustic wave device having an excellent electromechanical coupling factor can be obtained.
In a known method of improving the temperature characteristics, an AlN film which is a piezoelectric material is formed on an Si semiconductor substrate, and a film of silicon oxide is formed on the AlN film (U.S. Pat. No. 4,516,049).
All of these prior art surface acoustic wave devices have a lamination structure which is formed by one of various thin film forming techniques such as sputtering, and CVD methods. In this formation, combinations of a substrate and a material are strictly restricted. For example, a piezoelectric film formed by the sputtering method or the like is inferior in piezoelectric characteristics to a bulk single crystal. In order to exhibit piezoelectric characteristics, moreover, it is required to attain at least uniform orientation of the crystal orientation. However, such an orientation can be accomplished by a very restricted range of combinations of a substrate and a film. Preferably, a single crystal thin film is formed by an epitaxial growth technique. In this formation, combinations of a substrate and a film are further restricted. With respect to piezoelectric materials such as quartz, lithium niobate, lithium tantalate, and lithium borate which are used in conventional surface acoustic wave devices, for example, excellent epitaxial films have not been obtained when a different substrate material is used. Also in this case, therefore, there are problems in that the flexibility of design is low, and the types of materials with a large electromechanical coupling factor and a superior temperature dependence and excellent in sound velocity are limited.
With respect to these characteristics, it is known that a substrate having a large electromechanical coupling factor can be obtained by laminating a piezoelectric material having a large electromechanical coupling factor such as PZT, on a substrate of a dielectric or semiconductor material. Actually, there is no means for realizing such configuration. When one of the above-mentioned thin film forming techniques is employed, it is required to conduct the formation while orienting the piezoelectric material in a specified direction. Since combinations of the piezoelectric substrate material and a substrate are strictly restricted, no practical device has been obtained. When an adhesive is employed, the adhesive enters the interface for propagation of a surface acoustic wave so that the surface acoustic wave is attenuated, resulting in that preferable characteristics cannot be obtained.